Someone commented that a mission statement is no more than a fancy representation of a job description. A job description may be "digging holes as directed," but a mission may be as different as "sinking productive water wells,""building strong, secure fences," or "planting corn to feed healthy livestock." An organizational mission statement (its form is unique to each organizational culture) is an answer to the simple question, "What is the reason for my organization's existence?"
Being able to articulate one's mission is essential. Consider, for example, the Department of Homeland Security. It continues to struggle with this fundamental concept, in particular with respect to its relationship with FEMA. For more general reading on this topic, check out "Bureaucracy: What Government Agencies Do and Why They Do It" (Basic Books Classics) by James Q. Wilson.